Stagnation put off opponent (10)
I believe the answer is:
antagonist
'opponent' is the definition.
(synonyms)
'stagnation put off' is the wordplay.
'put off' indicates anagramming the letters.
'stagnation' is an anagram of 'ANTAGONIST'.
(Other definitions for antagonist that I've seen before include "Adversary" , "One who is hostile" , "Hostile person" , "5 down (anag)" , "Opponent" .)